The bug also happens with the wine glass in the Sanguine shop on Landslide. Poison it and the lady who drinks from it will die but the game does not register it as poison. A similar bug came up recently with Novikov: Poisoning the Bare Knuckle Boxer from the front of the bar registers as Poison, but doesn’t from the back.
I am 99.999999999% sure the bug is that “Consumed Poison” is a flag that is set when 47 interacts with the object to be poisoned, and must be manually flagged by IO’s map designers for each poisonable object in order to properly register. They missed some, or in the case of Landslide, missed a lot, which makes sense as Landslide has a ton of wine glasses.